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Northumbria Research Link Northumbria Research Link Citation: Morton, Richard, Weberg, Micah and McLaughlin, James (2019) A basal contribution from p-modes to the Alfvénic wave flux in the Sun’s corona. Nature Astronomy, 3 (3). pp. 223-229. ISSN 2397-3366 Published by: Nature URL: https://doi.org/10.1038/s41550-018-0668-9 <https://doi.org/10.1038/s41550-018- 0668-9> This version was downloaded from Northumbria Research Link: http://nrl.northumbria.ac.uk/id/eprint/37777/ Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University’s research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners. Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any format or medium without formal permission of the copyright holder. The full policy is available online: http://nrl.northumbria.ac.uk/policies.html This document may differ from the final, published version of the research and has been made available online in accordance with publisher policies. To read and/or cite from the published version of the research, please visit the publisher’s website (a subscription may be required.) The following article has been published in Nature Astronomy as: Morton, Weberg & McLaughlin, Nature Astronomy (2019) and can be found at: Doi: https://doi.org/10.1038/s41550-018-0668-9 URL: https://www.nature.com/articles/s41550-018-0668-9 Received: 28 February 2018; Accepted: 29 November 2018; 1 A basal contribution from p-modes to the Alfvénic wave flux in the Sun’s corona Authors: R. J. Morton1, M. Weberg1,2, J. A. McLaughlin1 1Department of Mathematics, Physics & Electrical Engineering, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK 2 National Research Council Research Associate residing at the Naval Research Laboratory, 4555 Overlook Avenue, SW Washington, DC 20375 Many cool stars possess complex magnetic fields [1] that are considered to undertake a central role in the structuring and energising of their atmospheres [2]. Alfvénic waves are thought to make a critical contribution to energy transfer along these magnetic fields, with the potential to heat plasma and accelerate stellar winds [3] [4] [5]. Despite Alfvénic waves having been identified in the Sun’s atmosphere, the nature of the basal wave energy flux is poorly understood. It is generally assumed that the associated Poynting flux is generated solely in the photosphere and propagates into the corona, typically through the continuous buffeting of magnetic fields by turbulent convective cells [4] [6] [7]. Here we provide evidence that the Sun’s internal acoustic modes also contribute to the basal flux of Alfvénic waves, delivering a spatially ubiquitous input to the coronal energy balance that is sustained over the solar cycle. Alfvénic waves are thus a fundamental feature of the Sun’s corona. Acknowledging that internal acoustic modes have a key role in injecting additional Poynting flux into the upper atmospheres of Sun-like stars has potentially significant consequences for the modelling of stellar coronae and winds. Alfvénic fluctuations have been observed regularly in the solar wind since the 1970’s [8] [9] and are typically considered to be of solar origin. Their atmospheric counterpart was inferred from the non-thermal broadening of coronal emission lines [10], but only within the last decade have studies of the Sun’s atmosphere been able to demonstrate unambiguously that magnetised plasma structures undergo displacements transverse to the magnetic axis [11] [12] [13]. Here we use infrared spectroscopic data (Fe xiii 1074.7nm emission line) taken from the Coronal Multi-channel Polarimeter (CoMP) coronagraph, which yield Doppler velocity time-series above the limb in the Sun’s corona and provide a measure of Alfvénic wave motions along a viewer’s line-of-sight [14] [15]. This is supported by extreme ultraviolet images of the corona from the 17.1nm (Fe ix) channel on-board the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA), which enables direct measurement of the transverse oscillatory displacements of the corona’s fine-scale magnetic structure [16] [17] (Fig. 1a, b, c). The data sets range between 2005 and 2015, hence covering various phases of the 11-year magnetic cycle, as the Sun’s global magnetic geometry undergoes substantial changes (Supplementary Table 1). Since they can be subject to unambiguous and detailed measurements, we utilise the observed transverse displacements of magnetised plasma structures to probe the flux of Alfvénic wave energy. 2 The power spectral density (PSD) provides a means to investigate Alfvénic waves and is straightforward to obtain from CoMP Doppler velocity time-series (see Methods). The PSDs show evidence for power law behaviour and display an enhancement of power around 4 mHz, sitting atop of the power law base-line (Figure 1d). This behaviour has been noted in previous observations of individual coronal regions [11] [14]. Significantly, recent magnetohydrodynamic (MHD) wave models demonstrate the potential for coronal Alfvénic modes to be excited at the transition region [18] [19] [20], resulting from a double mode conversion of the Sun’s internal acoustic (pressure or p-) modes that have leaked into the atmosphere through magneto-acoustic portals [21]. It is the observed enhancement of power that is considered to be the signature of Alfvénic waves generated by p-modes [11] [14] [20]. However, if p-modes are to play a crucial role in exciting coronal Alfvénic waves, their signature should have a spatially ubiquitous presence throughout the corona and over the solar cycle. Through examination of the Alfvénic waves associated with the power enhancement, we demonstrate that this is indeed the case. The counterpart to the velocity fluctuations measured in CoMP is thought to be the swaying motions of coronal structures observed in SDO/AIA [13] [17], but no direct comparison has previously been undertaken. To remedy this, we measured large numbers of oscillatory Alfvénic waves in SDO/AIA (see Methods), where the imaging observations project the transient, transverse motions of plasma structures onto the plane-of-sky (Fig. 1b). The new detailed analysis of SDO/AIA data reveal that the coronal wave properties are more complex than previously thought. A bivariate relationship is found between frequency and amplitude (Fig. 1c), with the periods and amplitudes occupying greater ranges of values than previously reported [16, 17]. The increased statistics permit a way to cross-calibrate the two sets of waves observations, enabling an estimate for the time-averaged wave properties of a particular coronal region and, hence, the PSD of the oscillatory motions in SDO/AIA data (Fig. 1d). The PSDs estimated from SDO/AIA data reveal the power spectra have a parabolic profile, which peaks around 3-4 mHz. Other peaks are also visible above the parabolic profile, but with the current uncertainties we cannot say whether these are genuine (See Methods). Comparison between the CoMP and SDO estimates for the PSDs of the Alfvénic waves reveals that the frequency location of enhanced power is congruous to the peak of the parabolic profile. Furthermore, the spectral indices from power law fits to both PSDs at frequencies > 4 mHz are in excellent agreement. The close relationship between PSDs suggests that the enhanced power in the CoMP data is due to the transient, oscillatory motions observed in SDO/AIA. Given this relationship, the enhanced power in the CoMP PSDs then provides a distinct marker for oscillatory Alfvénic waves and enables us to examine their nature throughout the corona and solar cycle. Our analysis of the CoMP data reveals that the enhancement exists in a large majority (>95%) of coronal power spectra, meaning that Alfvénic waves are present throughout the entire corona. An example of the key measured parameters obtained for 10 May 2014 data are shown in Figure 2. We obtain similar findings when extending to data from different phases of the latest solar cycle (Fig. 3 & 4), from 2005, two years before solar minima, to the decline from maxima in 2015. 3 The frequency corresponding to the centre of the enhancement is found to fall within a narrow range, with its distribution having a mean and standard error of 4.0± 0.1 mHz and standard deviation of 1 mHz (Figs. 2 & 3a). While the sample populations are small for each yearly data set, a comparison of the distributions from different years suggests there is little variation in the centre values over the solar cycle. The enhanced power is distributed over a broader range of frequencies, and the characteristic width has a sharply peaked distribution around 0.12 frequency decades (Fig. 3b). The ubiquitous presence of the enhanced power through the corona is further evidenced in PSDs averaged across all the coronal time-series from a single day (Fig. 4), where it is seen that the signature of Alfvénic waves is still clear. Aside from the enhanced power, the coronal PSDs from CoMP also display ubiquitous power-law-like behaviour, which indicates the presence of stochastic (or non- oscillatory) Alfvénic waves (Fig. 1d). However, it should be kept in mind we are examining a relatively short frequency range (0.1-10 Hz) and cannot determine if the coronal Alfvénic waves display scale-free behaviour. In spite of this shortcoming, we suggest the index from the fitting of a power law to the coronal PSDs can also provide insight into the nature of the coronal velocity fluctuations.
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